P481: FAILURE OF BONE MARROW ERYTHROPOIESIS IN ACUTE MYELOID LEUKEMIA IS DRIVEN BY LOSS OF ERYTHROBLASTIC ISLANDS
Authors: Laura Mosteo, Lidia Rocha, Ana Raquel Maceiras, Bruno Cavadas, Carolina Pereira, Marta Lopes, Sérgio Chacim, Inês Ramos, Edwin Hawkins, José Mário Mariz, Graça Porto, Delfim Duarte
Journal: HemaSphere (2023)

Background: Acute myeloid leukemia (AML) is characterized by blood cytopenias at diagnosis. The loss of non-malignant hematopoiesis in AML is driven in part by induced quiescence of hematopoietic stem cells, scavenging of growth factors, remodeling of the bone marrow (BM) microenvironment and space occupancy. AML patients often present with severe anemia, regardless of the degree of bone marrow infiltration. However, the mechanisms underlying the loss of erythropoiesis in AML are poorly understood. Aims: To investigate how erythropoiesis is affected in AML with a main focus on the study of potential changes in the microenvironment where these cells are generated. Methods: We used the well-established MLL-AF9+ AML mouse model and combined flow cytometry, imaging flow cytometry, erythroblastic island enrichment, RNA-sequencing and bioinformatic analysis for our studies. In specific experiments, erythropoiesis was also analyzed in a mouse model of Notch-driven T-ALL. We also analyzed primary samples of AML and control (lymphoma and MGUS) patients. Results: Through flow cytometry analysis we observed that AML progression is associated with a significant reduction in erythroblast subpopulations in the bone marrow. Particularly, more differentiated polychromatic and orthocromatic erythroblasts are lost at very low AML infiltration levels, whereas the earlier megakaryocyte-erythrocyte progenitors (MEPs) are only lost at high AML infiltration. These observations point to the decline of RBCs being due to a perturbation of the process of erythropoiesis downstream of MEPs. We validated an early loss of polychromatic erythroblasts in AML patients. The last steps of erythroid differentiation occur in the erythroblastic island (EBI) niche, which is composed by a central macrophage (nurse cell) surrounded by developing erythroblasts. We hypothesized that EBIs are disrupted in AML leading to loss of erythrocytes. After enriching for EBIs, we observed a decrease in erythroblast proliferation in mice with AML, consistent with the loss of EBI function. Imaging flow cytometry confirmed that native EBIs are significantly lost in AML. We next analyzed phenotypically-defined nurse macrophages, which were significantly reduced in mice and humans with AML, but not in mice with T-ALL. Transcriptomic analysis of nurse cells isolated from AML mice did not show changes in EBI-related or pro-inflammatory genes. In contrast, survival of nurse cells was impaired. Consistently, we detected a decrease in the pro-survival macrophage factor M-CSF in the BM of AML mice. Treatment of leukemic mice with M-CSF partially rescued nurse cells and BM erythropoiesis, suggesting that EBI units are depleted in AML as a result of loss of M-CSF. Summary/Conclusion: We unravel a new mechanism underlying the decay of erythropoiesis in AML. Previous studies demonstrated the important role of EBIs and nurse cell dysfunction in polycythemia vera, infection and myelodysplasia. We show that EBIs are also affected in AML, thus revealing a new cellular mechanism underlying the loss of non-malignant hematopoiesis in leukemia. Keywords: Macrophage, Acute myeloid leukemia, Erythropoieisis, Microenvironment